The present invention relates to the control of transmissions over a network by many stations or nodes all sharing that same network transmission path. Typical of such systems is the case where many nodes are coupled into the same optical fiber network and are capable of receiving the transmissions of all the nodes in the network.
Various systems have evolved to permit the nodes on such a network to transmit and receive in a manner that does not cause a collision between nodes, i.e., prevents two or more nodes from trying to transmit over the network at the same time.
One such scheme is known as time slotting. In this system, each node in the network is assigned a slot of time in a repeating sequence of plural slots, each typically of the same, unvarying size. This system is very inefficient in the use of network time because the time in each slot is committed to the corresponding station whether or not that station has any information to send. Only where all nodes on the network are always busy is there efficient use of the network capacity in such a system. The entry of or deletion of nodes from the network also requires complete rearrangement of the timing system, or the inclusion of extra time slots which waste network time.
A second type of time sequencing for enabling all nodes on a network to have an opportunity to use the network communication capability is token passing. In this case, a node grabs the network and keeps it until it is finished with the traffic it wishes to send. That node then sends a token or special indicia over the network which informs the next to send node of the opportunity for it to transmit. This system is not ideal for real time network usage, such as in live voice communication or real time control in robotics, because it does not insure each node a chance to transmit often enough to maintain the requisite data exchange rate.
A third system accommodating plural nodes trying to use the same network involves collision avoidance. Here a node desiring to send begins to transmit and continues unless it detects the presence of another node trying to send simultaneously. If this interfering situation is detected, both nodes cease sending and wait a different, randomly set interval before trying to send again. This approach is only efficient where there is little traffic from any of the nodes in the network. If communication is continuously underway, the time delays needed to resolve the collisions of nodes trying to send simultaneously is excessively wasteful.
A fourth approach to the simultaneous use of a single network by several nodes is the use of a master-slave hierarchy between nodes in which one node acts as controller for all the other nodes. Only in rare cases where the nodes are not fully independent is this system effective.
Finally, the grant-request approach of copending, commonly assigned application Ser. No. 534,562, filed Sept. 22, 1983, is an example of a further system utilizing a prioritization of requests for network use by all stations utilizing the same priority algorithm. Each station makes the same priority decision so that the node to send next is known to each node, including the node to send.